1. Field of the Invention
The present invention relates to a phosphor and a radiation image storage panel employing the same, and more particularly, to a divalent europium activated barium fluorohalide phosphor and a radiation image storage panel employing the same.
2. Description of the Prior Art
It has been heretofore known that a divalent europium activated barium fluorohalide phosphor absorbs a radiation such as X-rays with high efficiency. Also it has been known that the phosphor gives an emission (spontaneous emission) in the near ultraviolet region when exposed to a radiation such as X-rays, and that the maximum of the emission lies at the wavelength of about 390 nm. Recently, it has been discovered that the divalent europium activated barium fluorohalide phosphor is a stimulable phosphor. That is, when exposed to a radiation such as X-rays, the phosphor absorbs and stores a portion of the radiation energy, and emits light in the near ultraviolet region when stimulated with an electromagnetic wave within a wavelength region of 400-850 nm after exposure to the radiation. This emission is called "stimulated emission". Because of the stimulability thereof, the divalent europium activated barium fluorohalide phosphor has been paid much attention and investigated as a phosphor for a radiation image storage panel (a stimulable phosphor sheet) employable in a radiation image recording and reproducing method utilizing a stimulable phosphor.
For example, Japanese Patent Provisional Publication No. 55(1980)-12145 discloses a divalent europium activated barium fluorohalide phosphor showing a stimulated emission and a radiation image storage panel employing the same. The phosphor in the publication is expressed by the formula: EQU (Ba.sub.1-x M.sup.II.sub.x)FX:yEu.sup.2+
in which M.sup.II is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd; X is at least one element selected from the group consisting of Cl, Br and I; and x and y are numbers satisfying the conditions of 0.ltoreq.x.ltoreq.0.6 and 0.ltoreq.y.ltoreq.0.2, respectively.
The radiation image storage panel (stimulable phosphor sheet) employed in a radiation image recording and reproducing method described below has a basic structure comprising a support and at least one phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
As a method replacing the conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. The radiation image recording and reproducing method involves steps of causing a stimulable phosphor of a radiation image storage panel to absorb a radiation energy having passed through an object or having been radiated by an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission; photo-electrically processing the emitted light to give an electric signal; and reproducing the electric signal as a visible image on a recording material such as a radiographic film or on a recording apparatus such as CRT.
In the above-described radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of using the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
In the case that the above-described method using a radiation image storage panel is employed, particularly, for carrying out the medical diagnosis, it is desired that the sensitivity of the panel to a radiation is made as high as possible to decrease the exposure dose for patient and to facilitate the procedure for converting the stimulated emission to electric signals. Accordingly, it is desired that the luminance of stimulated emission of the phosphor employed for the panel is as high as possible. That is, a measure for enhancing the luminance of the stimulated emission as highly as possible is desired for the phosphor employed in the panel. Especially when the radiation is applied to a human body as described above, the enhancement in the luminance of stimulated emission is of much value from the viewpoint of adverse effect of the radiation on the human body, even if the level of the enhancement is not so remarkable.
Japanese patent application No. 57(1982)-166320 filed by these inventors describes a radiation image recording and reproducing method using a divalent europium activated barium fluorohalide phosphor containing a sodium halide having the formula: EQU BaFX.xNaX':yEu.sup.2+
in which each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I; and x and y are numbers satisfying the conditions of 0&lt;x.ltoreq.2 and 0&lt;y.ltoreq.0.2, respectively. The phosphor used in the radiation image recording and reproducing method is improved in the luminance of stimulated emission by introducing a suitable amount of sodium halide (NaX') into a divalent europium activated barium fluorohalide phosphor (BaFX:Eu.sup.2+) which is one of divalent europium activated alkaline earth metal fluorohalide phosphors.
Further, the above-mentioned effect of the added NaX' to enhance the luminance of stimulated emission of the resulting phosphor has been confirmed in a divalent europium activated strontium fluorohalide phosphor (SrFX:Eu.sup.2+) as well as in a divalent europium activated calcium fluorohalide phosphor (CaFX:Eu.sup.2+). In other words, it has been confirmed that a divalent europium activated alkaline earth metal fluorohalide phosphor containing a sodium halide and having the following formula, exhibits the stimulated emission of higher luminance than a divalent europium activated alkaline earth metal fluorohalide phosphor containing no sodium halide: EQU M.sup.II FXxNaX':yEu.sup.2+
in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I; and x and y are numbers satisfying the conditions of 0&lt;x.ltoreq.2 and 0&lt;y.ltoreq.0.2, respectively.
In the aforementioned radiation image recording and reproducing method, there is a problem that the resulting image is degraded by a noise to lower the image quality when a radiation image storage panel is left for a long time or the panel is repeatedly used for a number of times. When a radiation image storage panel is left to stand for a long time, the stimulable phosphor of the panel absorbs and stores the energy of radiations such as radiations emitted from radioisotopes such as .sup.226 Ra and .sup.40 K contained in the phosphor in a very small amount and environmental radiations. Thus stored radiation energy causes the noise to lower the image quality. Otherwise, the radiation energy stored as a certain kind of latent image in the panel upon exposure to the radiation passing through an object is partly released as an emission upon stimulation but a portion of the radiation energy in certain cases remains in the panel even after the stimulation is carried out, so that the residual radiation energy causes the noise in the subsequent use of the panel. The noise particularly caused by the latter radiation energy remaining in the panel is apt to be remarkably observed on the resulting image, in the case that the exposure dose of a radiation is small in the subsequent use of the panel, that the radiation image recording and reproducing method shows a high sensitivity, or that the stimulation is carried out under the insufficient conditions such as that the intensity of the stimulating rays is low or that the wavelength of stimulating rays is not proper for the phosphor used therein.
As a method for erasing the stored radiation energy causing a noise from a radiation image storage panel, Japanese Patent Provisional Publication No. 56(1980)-11392 discloses a procedure which comprises exposing the panel to light having a wavelength within the region of stimulation wavelength of the stimulable phosphor thereof prior to exposure to a radiation such as X-rays to be absorbed. For applying said erasing method to a radiation image storage panel, it is desired that a stimulable phosphor employed in the panel has the high erasability, that is, the decay of stimulated emission thereof is as fast as possible upon stimulation with an electromagnetic wave having a wavelength within the region of stimulation wavelength thereof.